Electrical measuring instrument



p 1946. a. B. GRYA'CEI ETAL' 2,408,060

ELECTRICAL MEASURING INSTRUMENT Original Filed June 12, 1943 8 J 6214:.Bv (ii/6 0 A r Attorney Patented Sept. 24, 1946 2,408,060 ELECTRICALMEASURING INSTRUMENT Bertram Barnett Grace, John Handley, and

Gilbert Gilliver, London, England, assignors, by mesne assignments, toInternational Standard Electric Corporation, corporation of Delaware NewYork, N. Y., a

Originalapplication June 12, 1943, Serial No.

490,627. Divided 29, 1943, July 17, 1942 6 Claims. (01. 17195) Thisinvention relates to electrical measuring instruments of the moving coiltype in which the permanent magnet is situated within the moving coilwhile a ring of magnetic material surrounds the magnet and coil assemblyto define the outer periphery of the air gap in which the coil swingsand to complete the magnetic system, and is a division of our co-pendingapplication Serial No. 490,627 filed June 12, 1943.

The invention consists in using for the permanent magnet on an elementof an alloy of high BH (max) which has been subjected to a steadymagnetic field during cooling and in assembling the parts so that thedirection of maximum magnetisability of said element as determined bysaid treatment during cooling shall be coincident with the direction inwhich the element is then magnetised after assembly. 7

The invention will now be described with reference to the accompanyingdrawing, in which:

Fig. 1 is a front View with cover and scale removed; and

Fig. 2 is a top plan view with the base or rear cover shown in sectionon the centre line.

The framework of the mechanism consists of a ring-shaped-pole-piece 19to which two die cast supports I 4 are rigidly attached by rivets 24.

The central permanent magnet I1 is cylindrical in shape but with a flatportion IT on one side and it is fitted within a cradle of non-magneticmaterial comprising a pedestal 2| and a pair of diametrically oppositepole shoes IS the pedestal being adjacent to the fiat portion on themagnet and insuring that the magnet is assembled in a definite angularposition. When assembled the magnet is magnetised across its diameter,the magnetic axis being parallel to the flat portion and correspondingto the direction of the magnetic field in which the material was heattreated during manufacture. Reference may be made for example to thearticle in Nature for July 30, 1938, page 209 for such special processof magnetisation. The magnet and pole shoe assembly is held in the ringby the pedestal, which is located in one of the recesses [9' in the ringand secured by the screw 23.

The outer ring I9 is concentric with the magnet and its polepieces, sothat an arcuate air gap I8 is formed between them with the centre of thetwo opposite arcs coincident with the axis of the cylindrical magnet.

The moving coil 20 embracing the magnet is of rectangular shape so thatits opposite sides 7 swin in the air gap I 8 on its pivots which arecoincident with the geometric axis of the magnet and outer ring.

one pole and this application November Serial No. 512,176.

In Great Britain The pivot bearings, control springs and adjusters aremounted on the two bridge pieces 38 which are attached to the frameworkby the screws 39. The bridge pieces are insulated from the frame bybushings and washers.

The open ends of the pole shoes l8 are extended towards one another at areduced width and machined off fiat on their extended portions. Ashort-circuiting screwed plunger 48 is provided to short circuit orshunt the magnetic flux from shoe to the other and so reduce the fluxdensity in the gap for the purpose of finally adjusting the deflectionof the instrument.

The framework isiattached to the base ID of the instrument by fourscrews 42 which are screwed through the base into lugs 4 2A on the rearend of the die castings M,

The scale plate 35 is attached to lugs on the front of the die castingsby screws 4|.

The permanent magnet I1 is made from material havin a very high BHmax,where B is the remanence, H is the coercive force and BHmax is themaximum product of these two values. The best of such materials atpresent known are alloys of aluminium, nickel and cobalt, which havebeen so heat treated in a magnetic field as to enable a much higherBHmax to be obtained, when subsequently magnetised in the samedirection. This gives a very high flux density in the arcuate air gapsbetween the pole shoes and the ring. One such permanent magnet materialis sold under the trade name of Ticonal 42/50.

The proportions of the magnet system are so chosen that the magnetproduces its BHmax, a condition that is dependent on the relationshipbetween: the cross sectional area of the magnet S, the average length ofthe flux path in the magnet L, the cross sectional area of the flux pathin the gap s, the combined length of the flux paths in the two gaps Z,and the leakage factor f.

The proportions are then given by For Ticonal 42/50, BHmaX=4-22 x 10where B=8600 lines per sq. cm. and H=490 Gilberts per cm. so that B 8600F T -17.5 The dimensions of the magnet system therefore are soproportioned that 3, 17.5 approximately The width of the air gap is nomore than is necessary to accommodate the coil with working co "and ponter, and, a suitable swing of :the p I diameter and length of magnetand the leakage factor are calculated to give B/I-I.

It will be understood the magnetic flux is not to beuniformlydistributed throughout the length ofthe gap which may beattained by non-circular curvatureucf the poles or upstanding portionsofthe outer ring or by these surfaces the axis of the magnet and pivotsand producing air gaps of varying widths.

What is claimed is:

1. An electrical measuring instrument comprising a permanent magnetconsisting of cylindrical element of an alloy of iron, aluminium, nickeland cobalt which element has been sub. jectedlduring cooling to a steadymagnetic field across a diameter and has been permanentlyniagnetisedalong, said diameter; a pair of cylindrical-arcuatepole-shoes of high permeability material 'overlyingthe-poles of themagnet; a cradle oiv-non-magnetic material to which the pole-pieces aresecured in such positions that they grip the magnet between them; acontinuous ringer high permeability material urroundingthemagnet-and-pole-shoq assembly coaxially ti'ierewit-h and formedwithupstanding portions in juxtaposition to the pole-shoes to definetherewith thatwo opposite arcsof a cylindrical-arcuate air-gap; and amovingcoilembracing the magnetand pole -shoe assembly and, pivotingcentrally about an axis coincidentwith the axis of the magnet-system;the magnet-and-pole-shoe V assembly being accurately located withrespect to the-continuous fring by the seating of the cradle inra gapbetween the upstanding portions of the ring 2-. An,electricalmeasuringinstrument, comprising a moving coil and a permanentmagnet within said coil, said permanent magnet consistingofgan elementrendered =magnetically anisotropic during heat treatment and permanentlymagnetized in substantially one direction, said directionbeingsubstantially perpendicular to that of the path oi said moving coil.

the correct ratio of.

being eccentric in relation to nonmagnetic pedestal and a pair of thatthe invention. may I take other forms an example being that in which 3.An electrical measuring instrument, comprising amoving coil, apermanentmagnet withsaid coil, said permanent, magnet consisting of anelement rendered magnetically anisotropic during heat treatment, acradle comprising a diametrically opposite pole shoes surrounding saidpermanent magnet and means carried respectively by said .permanent'fmag'net. and said pedestal to locate th permanent magnet inthe correct angular positionto .haye it magnetized in a field in thesame directionasthe held in which it was heat (waned, said directionbeing substantially perp'endi cular to that of the path of said moving i4. An electrical measuring instrument, comprising a moving coil, acylindrical permanent magnet within said coil, said magnet having a flatportion on one side thereof and consisting of an element of .an alloy ofhigh Bl-l max which has been subjectednto a'steadymagnetic field duringcooling, 2, cradle comprising a non-"magnetic pedestal and. a pair of,arcuate'pole shoes overlyingthepoles of the permanent magnet and a flatsurfaceonisaid pedestal engageable with the fiat portioniof tlielpermanent, magnet to locate the magnet inithe, correct angularposition to have it magnetized ina field in thesame direction asthefield in .whichit was heat treated, said directionbeing substantiallyperpendicular to. that V of thepathof saidmovingcoil, and said flatsurface being substantiallyparallel to said directien of" magnetization.

5: Anins trument accordingto claim 2, wherein said. directionof-magnetization is substantially perpendiculartothe direction'of thepath of said moving coil in its medium position. i

6. An instrument according to claim 2, wherein said .movingcoil is,arrangedirotatably about an axis, saidlpermanent magnet has pole piecesforming a cylindrical air gap to receive said moving coil, and saiddirection of-magnetization is substantially perpendicular to the axis ofsaid moving coil.

BERTRAM-BARNETT GRACE. JOHN I-IANDLEY. GILBERT GILLIVER,

